1. Field of the Invention
The present invention relates to a frequency modulating circuit. More particularly, it relates to a frequency modulating circuit which is automatically controlled without an operator for controlling a carrier during frequency modulation.
2. Description of the Related Art
A video tape recording (VTR) system is capable of recording an audio signal and video information for replay when desired. In this system, video signals are stored as signals having a modulated frequency in recording media such as magnetic tapes.
When the video signals are stored on a magnetic tape, luminance signals are stored on the magnetic tape after frequency modulation. A frequency modulating circuit generally includes an oscillator formed of a resistor and a capacitor and a control system mounted outside to control the desired carrier frequency necessary for proper frequency modulation. The carrier frequency is externally adjusted with the use of a variable resistor. This external resistor produces a voltage control of the frequency of the oscillator. The variable resistor control and resistor/capacitor oscillator form a voltage controlled oscillator (VCO).
In general, a VCO is used with a phase detector that compares a reference signal of a predetermined carrier frequency with an output of the VCO which would otherwise oscillate freely. A phase difference is detected by the phase detector, which generates an error voltage corresponding to this phase difference. The phase difference is fed back to the VCO to control the output of the VCO to the reference frequency.
The frequency modulating circuit of the VTR conventionally has a VCO which modulates an input signal which is to be recorded. The carrier frequency of the frequency modulating circuit is controlled by using the error voltage obtained by the control loop of the VCO.
Conventionally, the frequency of the carrier frequency is precisely controlled by the manual adjustment of an external variable resistor. However, this conventional precision control of the carrier frequency has the disadvantage of raising the costs of production of the VTR. In addition, the manual adjustment of the variable resistor increases the probability of error in the control.
Further, there is the need for a circuit which generates a 3.4 MHz oscillating signal to be used as a reference signal. Also, the phase detector may generate a large amount of error in the circuit if the carrier frequency increases.